Limited supply of fossil energy resources and their associated global environmental damage have compelled market forces to diversify energy resources and related technologies. One such resource that has received significant attention is generating energy through electrolysis of water.
As such, extraction of hydrogen from water continues to attract substantial attention as a clean form of energy (e.g., for fuel cells). In general, electrolysis of water can be obtained by passing direct current from a battery through a water container, wherein presence of acid/base salts increases the reaction intensity. Using platinum electrodes, hydrogen gas will bubble up at the cathode, and oxygen will bubble at the anode. If other metals are used as the anode, there is a chance that the oxygen will react with the anode instead of being released as a gas, or that the anode will dissolve. For example, using iron electrodes in a sodium chloride solution electrolyte, iron oxides will be produced at the anode. With zinc electrodes in a sodium chloride electrolyte, the anode will dissolve, producing zinc ions (Zn2+) in the solution, and no oxygen will be formed. When producing large quantities of hydrogen, the use of reactive metal electrodes can significantly contaminate the electrolytic cell—which is why iron electrodes are not usually used for commercial electrolysis. Moreover, electrodes fabricated from stainless steel can be used because they will not react with the oxygen.
Accordingly, solar hydrogen generation represents a promising long-term objective for the energy industry. Moreover, efficient, low-cost methods of generating hydrogen from renewable solar energy remains an important element of the future hydrogen economy. With clean and abundant, photoelectrochemical, or photocatalytic hydrogen generation could become viable technologies. However, to make this a reality, it is necessary to reduce costs, increase efficiency, and improve service life.
For current solar photovoltaic cell-driven electrolysis, the overall efficiency is the product of the efficiency of the photovoltaic cell and the efficiency of the electrolyzer. Photovoltaic cell efficiencies have been reported from 6% to as high as 32% with different materials. Current electrolyzer efficiency is approximately 75%. Hence photovoltaic cell-driven electrolysis efficiency could be from 4.5 to 24%, while in practice values at the low end of this range are encountered. Such low efficiencies are in part due to efficiency losses from sunlight absorption by a liquid electrolyte layer, impediments to the departure of product gases from the photo electrodes due to electrolyte surface tension, and high over potential of the photo electrodes. In addition, system life is limited by photo corrosion and electrochemical corrosion of the electrode. Further, costs of such devices remain too high for wide use.